1. Field
The following relates to an arrangement for increasing a relative difference in resistance of a magnetoresistive memory cell having in each case a memory layer and a reference layer on both sides of a tunnel barrier between a first magnetization state and a second state.
2. Background Information
A magnetoresistive memory cell usually comprises two ferromagnetic systems with a non-ferromagnetic isolating layer situated in between. In the simplest case, the two systems comprise a ferromagnetic layer in each case.
The first ferromagnetic layer is typically made of a hard-magnetic material, for instance a cobalt-iron alloy. This ferromagnetic layer, with a constant magnetization in terms of magnitude and direction, functions as a reference layer.
The second ferromagnetic layer made of a soft-magnetic material, typically a nickel-iron alloy, forms a memory layer. In a manner corresponding to a data content of the memory cell, the magnetization of the memory layer is oriented unidirectionally or oppositely directed with respect to the magnetization of the reference layer. The memory cell thus has two distinguishable magnetization states (unidirectional, oppositely directed) in accordance with its data content.
If the magnetoresistive memory cell is based on the tunnel effect, then the material of the isolating layer is a dielectric. The frequency of a transition of electrons from one ferromagnetic layer to the other is higher in the case of unidirectional magnetization of the two ferromagnetic layers than in the case of oppositely directed magnetization of the two layers. From the conductivity of the layer system, it is possible to deduce the orientation of the magnetization of the memory layer relative to the magnetization of the reference layer and thus the data content of the memory cell.
The more pronounced the difference in conductivity or electrical resistance for the two magnetization states of the magnetoresistive memory cell, the higher the degree of interference immunity with which, and more simply, the data content of the memory cell can be read out. The difference in resistance behavior becomes greater, the fewer magnetic domain regions the two ferromagnetic layers have and the higher the spin polarization within the two layers.
In the known art, the difference in resistance for the two magnetization states is 15-20%, for example, in the case of magnetoresistive memory cells based on the tunnel effect. In a semiconductor device having magnetoresistive memory cells, by contrast, the resistance of two adjacent memory cells in the semiconductor device with the same magnetization state can also be significantly more than 20%. The conductivity thus diverges between two memory cells of equal magnetization with the same order of magnitude as between the two magnetization states of a memory cell. This makes it considerably more difficult to evaluate the magnetization state and assess the data content of a memory cell.
In conventional concepts for the reference layer of a magnetoresistive memory cell, the reference layer is designed as a magnetically hard layer which receives its magnetization in the course of a fabrication process for a semiconductor device having a magnetoresistive memory cell and essentially retains it for an entire service life of the semiconductor device. Temperature and long-term data stability of the magnetoresistive memory cell depend directly on the stability of the magnetization of the reference layer.
In present concepts, the reference layer is either coupled (pinned) to natural antiferromagnetic layers or supplemented with at least one further ferromagnetic layer of oppositely directed magnetization to form an artificial antiferromagnet. Ferromagnetic and antiferromagnetic systems that are coupled to such an extent via the Rudermann-Kittel-Kasuya-Yoshida (RKKY) interaction have an improved temperature and long-term data stability compared with individual hard-magnetic layers and are less sensitive to interfering magnetic fields.
FIG. 3 illustrates a diagrammatic cross section through a magnetoresistive memory cell. A reference system 6 and a memory system, which in this case comprises an individual memory layer 1, lie opposite one another on both sides of a tunnel barrier 2. The reference layer 3 is a sublayer of the reference system 6 which is oriented towards the tunnel barrier 2. In this case, the reference system 6 is fashioned as an artificial antiferromagnetic layer system (AAF), comprising the reference layer 3 and a reference coupling layer 5 on both sides of a spacer layer (a spacer) 4, a magnetization 9 of the reference coupling layer 5 being oppositely directed with respect to a reference magnetization 8 of the reference layer 3.
In a similar manner, the reference layer 3 (then as pinned layer) can be coupled via the spacer 4 to another layer (pinning layer) made of a naturally antiferromagnetic material. Such concepts are known from magnetic sensors and, in particular, from magnetic read/write heads.
A plurality of coupling mechanisms, for instance Néel interaction (orange peel coupling), pinhole coupling and interactions via magnetic leakage fields, act between the reference layer 8 or the reference system 6 and the memory layer 7.
Layers that are strongly coupled to one another react more sluggishly with respect to a magnetization reversal than weakly coupled systems. Therefore, present-day concepts for reference systems attempt to reduce the magnitudes of these interactions in order to obtain better dynamic properties of the magnetoresistive memory cell.
Secondly, a bias field resulting from the sum of the interactions, in the memory layer of the magnetoresistive memory cell, leads to an asymmetrical switching behaviour of the memory layer. Therefore, the present concepts attempt to achieve compensation of the magnetic coupling mechanisms at the location of the memory layer.
To summarize, the following, partly conflicting requirements result for the reference layer, or a reference layer system: (i) temperature stability, long-term data stability and magnetic field insensitivity each require a thick and magnetically hard reference layer; (ii) a low magnitude of the Néel coupling requires a thick memory layer; and (iii) a symmetrical switching behavior of the memory layer presupposes a reliably reproducible surface roughness of the reference layer and an adjustable leakage field.